RE: Adding AR Aging invoices into GP from acquisition of new company
For referential integrity you should first integrate the customers, then integrate and post the open AR documents.
I've always used Integration Manager to do this and for new implementations that's pretty standard. To bring in a mass of documents into an existing AR would still be the same process but more likely to have issues to be aware of. Duplicate customer records / expense account mapping issues / posting period / shared AR or different account or date issues etc... On some installs where open AR was not a huge number I've seen customers continue to record cash in the old system then do bank transactions to balance cash into the new system.
What works is posting it into your GP system, what probably wont fly is trying to back end it into your system.