But i need to do the reverse. Create a form template from the files in a
folder. And i want to add a dll. I partially solved the problem by adding
the dll as a resource file in the infopath form(in design mode) but this
way, the form code isn't able to execute the api's in the dll because it
doesn't have full trust.

Any other way i could do that?

PS: The constraint is that we don't have codebehind at this stage. Some
other person should be able modify the xsn file to add a dll somehow.
(Assuming he has the private key to sign the form again.)