Well, if you have the talkgroups in another format, you can probably just copy/paste that into the submission text box. As of right now, no, there is no provision to allow attaching documents to submissions. That would probably be somewhat counterproductive, as it would encourage some to submit "data dumps" of documents they found elsewhere on the web.
Remember, submissions are only supposed to contain personally verified information, not a dump of something found online. If there is an official programming document or something of that level, that might be accepted after verification of the accuracy and legitimacy (believe it or not, not all official documents are 100% accurate or up to date). In that case, if copy/paste is not possible for some reason, you might try contacting a local admin privately via a PM. Worst case, where you cannot copy/paste the data and are unable to contact an admin for that area, you can make a submission asking that you be contacted directly via email.