If the logged-on user on the workgroup machine has local admin rights then you don't need to embed any credentials into the agent installation package you are building.
could you enter \\localhost\<username> ?
Firstly, as Rob said, if your users have local admin rights then you can use the normal framepkg - that's the easiest approach
If you do need to use a package with embedded credentials, please make sure that you have checked the embedded credential version of the agent into the master repository. (Because of the huge size difference between a package that supports embedded credentials and one that does not, we release two versions of the agent.)
Lastly, you used to be able to use . to represent the local machine - so .\Administrator would resolve to %COMPUTERNAME%\Administrator. I haven't tried this recently though - give it a try and see what happens