It is possible. I am starting to learn this myself and beginning to figure it out, but if you are not careful you can crash your game. Look in your lspdfr/data folder. There are a bunch of XML files in there that can be edited to get what you want. In addition, you will see a custom folder where you can place custom agency files. It took me a lot of trial and error to get this working right, but now that I am starting to figure it out, I am starting to get it to what I like.
My first suggestion is to backup all those files in the lspdfr/data folder and lspdfr/data/custom folder if it exists. What files are in these folder will vary if you have EUP installed (recommended) because EUP will also try to match the uniforms with the cars.
The folder should have XMLs for Regions, Backup, Duty_Selection, inventory and stations. All these can be edited for a more immersive experience, but it can get overwhelming so let's start small. The regions file defines what "zones" are in each region. By default there are regions for Los Santos City, LS County,Blaine County as well as a some of the special regions like the jail, airport, parks, etc.
My suggestion to get started and how I got started, it to try to get custom cars/peds patrolling Blaine first since it is already defined as a region.
Look in the custom folder for a XML titled agency_bcso.xml. If you have it open it up and look at it. That is where you can define what cars patrol Blaine County. Substitute sheriff and sheriff2 for your custom cars. That alone should get them patrolling Blaine County. I would start with that simple change first but once you learn how these files work it can get extremely interesting.
Again BACKUP YOUR FILES FIRST. I messed these up several times experimenting and LSPDFR will crash if these files aren't right. But crashing is how I started to learn how these files work.
I am still learning myself, but will be happy to share what I learned. Final note, I have Ultimate Backup installed too. UB has additional files and complicates everything. You can still change the custom agency file and get the cars to patrol ambiently and they will respond as backup from the default backup menu, but won'r respond right if you use the Ultimate Backup menu.
To get started, I recommend you download Jurisdictions+ from here but don't use the DLL, its not needed. Just look at the XML files for the various options. Study them to see if you can begin to understand how they work. That's how I got started. They are well annotated and should help (these files assume EUP and Ultimate backup)
https://www.gta5-mods.com/misc/lspdfr-xml-jurisdictions-versions
You can install the map only OIV file and you map will be updated to show the Jurisdictions. I myself use the option 4 map. I don't recommend installing the entire option as an OIV. I think it's better to suffer the pain and do it manually so you can understand what is going on.
Figuring this all out did give me quite a headache, but I am very pleased with how it is coming along and I am still working on it.
As an example of how complex you can get this
So far I have Port Authority Police patrolling the airport and Terminal area (no need for two separate department here).
Paleto Bay patrolling Paleto Bay
Blaine patrolling all of Blaine, including Paleto (50/50 with Paleto)
LSSD patrolling just their county (sometimes they wander into the dessert too)
University PD patrolling Richman along with LSPD (its really just 1 or two streets but it looks cool)
Park Ranger patrolling Mt. Chilliad with a 15% chance they can show up as backup in Paleto Bay and a 70% chance that Blaine will back up the ranger
SAHP patrolling all the highways
Blaine SWAT and K9 will respond in Blaine including Mt. Chilliad
I am going to create a Grapeseed PD and then create a chance they can show up as backup in Sandy Shores (Mutual Aid)
But I found out the hard way. A simple math error in setting the probabilities and it crashes.
So far I have just been working on the cars with default uniforms so for now SSPD and Paleto use BCSO uniforms, but Port Authority has PAPD uniforms thanks to EUP. Once I get brave and more competent, I will start to give each department their own uniform.
To be honest the only thing that is missing is I wish there was a way to limit the number of units from a single agency. It would be really great if I could limit Sandy Shores to just 2 units and if I requested more backup it would come from the county or Grapeseed.