I'm not sure I understand how testrpc accounts work.
Apologies. I missed the title and voted to close.
TestRPC doesn't regard any running blockchain.
You can think of testrpc as a fast RPC interface emulator for development purposes. It's an alternative to a node. It isn't a complete implementation and the state isn't persistent between restarts, but it's very convenient for development.
I usually do:
$ testrpc -u 0 -u 1 to unlock the first two random test accounts (with Ether) that it conjures up.
Hope it helps.