This actually should work. You just need to copy the content of the pem file to the private part of the key when copying it into Termius app in the Keychain section.
We’ll double-check it.
If you have any other problems please contact us here – http://support.termius.com/
I tried on Mac OS, it still don't work with pem file after I added into Performances/KeyChainTommy commented
Can't find a way to add pem file into Termius Mac version, ideally, the user should able to drag pem file to the keychain in the performance of Termius Mac app. hope it will be supported soon.