Hi,
There is much on SDN for legacy data upload planning. To your query in nutshell, create following GLs:
1. For customer -- use "Initial Customer Balance Upload" as GL
2. For Vendor -- use "Initial Vendor Balance Upload" as GL
3. .....
This Initial GLs are retreated as contra entry while uploading and will be totalling to zero amount in ledger eventually as your trial balance is tallied.
BR
Firoz S