That could be tricky. Off the top of my head I think you can do the following (in theory, not tested, mileage may vary, all sales final)

1. Use a thick client designer such as K2 Studio or K2 for VIsual Studio.
2. In your process that you want to crunch the data, use an advanced destination rule and set it as plan per slot. the list method the slots will require is a list of record IDs from your source DB. Hopefully you have these IDs ahead of time to avoid extra calls. (NOTE: to get the the advanced designation rule, access the destination rule options from the activity property, click the back button and select the "Advanced Mode" checkbox which will expose more options.)
3. 
4. You will need two Smart Object Events in your activity. One for source data and one for destination. You will want to make the return of your source smart object event call to an activity level data field. Since you will be doing a loop you want each cycle of the loop to have it's own data which is why process data fields won't work here as you will step on yourself.
5. the destination smart object event will use the activity data fields above as input properties.

Hope this helps.

S.

One parting thought, on your activity data fields, ensure that they all have "Keep Audit" turned off (In the screen shot below it is turned on which is default behavior). Depending on the number of records you don't want to go filling up your ServerLog schema with a bunch of transient data.

Thanks Scott!  That worked.