In most cases, they're not going to be purchased loans (unless your "merger" transaction was really strange). If you truly did just purchase the other bank's portfolio (and didn't acquire the whole entire institution) - then you have to report every single reportable loan in their portfolio - not just the current year stuff. That is a potential nightmare that you want to avoid.
The two guides (Getting It Right) list your options for reporting when two institutions merge. They give you several scenarios to choose from as well. When we acquire a bank, we usually choose to merge their HMDA LAR into ours and report as one bank. Same with their CRA LR. If the acquired institution isn't a HMDA reporter, or a CRA reporter, then we typically just pick up their activity starting with the acquisition date.
You might choose another of the options available - you just have to determine what is best for your particular situation.
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Opinions my own.