Can tempered chocolate be seeded with a different chocolate?
You cannot use sugar unless you do not mind having crunchy sugar crystals. There is no way of sweetening chocolate by adding sugar, even powdered sugar, without a conche. I think if the cacao contents are not too dissimilar, you should be fine. I have got away with 70% seeding 85%, but I cannot tell you whether you could get away with any mix. You can also sweeten your 100% by mixing sweet dark chocolate.

I would not use whole chips though, you want it grated. In fact, I would temper the Callebaut first and grate it. Keep a stash of that as seed. I have also routinely tempered dark chocolate without seeding.