Supernetting a Class B network, Can it be done?

Currently we have been assigned two consectutive subnets on a B Class network (130.15.128.0 and 130.15.129.0). The 130.15.128.0 subnet is our primary network and is assigned a 255.255.255.0 netmask. The 130.15.129.0 subnet has been divided up into smaller variable sized subnets (we have 130.15.129.224/30, we have 130.15.129.241/29 and 130.15.129.209/29 and 130.15.129.129/28 ) by my predecessor. The 130.15.128.0 subnet currently is at 80 % capactity, and the only addresses used on the 130.15.129.0 subnet are the ones I indicated above.

My problem is as follows, we are moving to a larger building and at the same time are increasing the number of PC's on the network. My initial plan was to utilize the unused first portion of the 130.15.129.0 subnet for these new pc's in the new building and keep the existing pc's on the 130.15.128.0 subnet and when ready to move completely into the new building take the network down in the old building and move it to the new building all in one fell swoop. Unfortunately my supervisors don't want the network down at all and want to move people and pc's one office at at time over a 6 -8 month period as the new building gets finished. So now I will have some pc's on the 130.15.128.0 in both locations and the 130.15.129.0 pc's in the new building only, but all will need to access the internal networks and connect to the external networks as well. I only have three Cicso 3640 routers to work with to get this setup done. One being a spare and the other two connecting us to three seperate networks at other company locations. I will be using the spare router in the new building as an initial connection to the existing network, I have looked into the "Supernetting Idea" which sounds like it will do what I want it to do, but am not sure if this will work with my setup, especially the variable subnets on the 130.15.129.0 subnet. Does anyone out there have any suggestions or experience with supernetting that could help me figure this out?

Could assigning one of the ethernet ports on the third router (the one in the new building) with an available 130.15.128.x/24 IP do the trick?