In theory you can do that. Add the ERCs into ESM, apply the data source configuration to the ERCs and then physically move them but still keep them into ESM.
Depending on your ERCs you have different values for internal storage (McAfee Event Receiver – Event Collection | Intel Security Products ) which you should take into account.
It's gonna be interesting when you connect them after a while and the ESM starts pulling data :-D
Let us know how it goes.