Not 100% sure TBH. You could try the same import process as here:
https://suiteanswers.custhelp.com/app/answers/detail/a_id/105270/loc/en_US but doing an "Add" versus update. Although it's very possible proposals have to have a transaction associated with it, so you wouldn't be able to import a (standalone) asset proposal. You WOULD be able to import a JE or other transaction(s) that hit the asset account to get a proposal to generate (this is how I usually do it).