** I know this is a super late reply which might be maybe useful for other readers.

Looking at your original description, you are either not explaining it right or you have a problem in your design/DW concepts. The key for a dimension is supposed to be a surrogate key and has nothing to do with the source PK. So, when you say you have to make the dim key a varchar I am not sure what you actually mean by that.

As for the issue in SSIS, I haven't tested this, but I believe you will need to create a Default constraint on your dimension key which will basically call the NEXT VALUE FOR function on your sequence object. This way SSIS will not insert into that column and the default will take care of it (similar to how an identity column would work).