Yes, you can!
All you have to do is add $5 PER EXPANSION to any pledge of $25 or greater. For example:
If you wanted to pledge for HOUSE PARTY (one copy of Belle to the US) and one Sideshow expansion, your total pledge would be $30.
If you wanted to pledge for HOUSE PARTY and one Sideshow expansion and one Badlands expansion, your total would be $35.