It's possible to add templates you or your colleagues have created to the app. The template file will be a .itmp file (no other format will work).
The easiest way to get the template file onto your device is to attach it to an email to yourself. Once sent, open this email up on your iPad and then tap on the attachment. You should see a window open up that will show a range of options, with "Copy to Inspector" being one of them. Tapping that will open the template within Inspector and save it to the app for future use.
If you don't see the "Copy to Inspector" option, tap on the "More" option along the first row of options. The following screen will list every possible option, as well as the ability to re-order them. Tapping the switch next to the Inspector option will ensure that it appears in the first screen in the future (tap Done in the top right to confirm this).
Note: the above instructions are for the native Mail App on iOS. If you use a third-party email app, you may need to complete some additional steps before you're able to open the file.