Hi, to add a new holster to CopHolster you need to open the XML and scroll down and add a new line to the MP_M_FREEMODE_01 slot (This will be down the bottom) once here you can see a bunch of already added holsters under <weapons> that look like.
<Item WeaponSet="Pistols">
<Component>8</Component>
<Holstered>16</Holstered>
<Unholstered>18</Unholstered>
</Item>
Now make sure to change the component slot to 7 otherwise it will not be in the correct slot. to add a new one copy and paste this line just below the one at the bottom and match up the numbers of the holster in game to the numbers in the XML but take note that if your using EUP Menu usually you need to -1 to the item you wanna add for example 134 becomes 133.
For further example if I wanted to add a holster that's in slot 100 (Holstered) and 101 (Unholstered) it would look like this
<Item WeaponSet="Pistols">
<Component>7</Component>
<Holstered>99</Holstered>
<Unholstered>100</Unholstered>
</Item>
Hope this helped and if you have any more questions just hit me up!