I would do it in following steps:
1. be sure that a spanning tree protocol is enabled
2. I would disable the 6 new links on each switch
3. I would add the 6 links to the trk1, I dont use lacp just trunk, it works fine and is protocol independent
4. Now u have to add all vlan config form the orginal link to trk1, on each switch
5. I would connect the links physically
6. I would enable the links, initially on the switch which isnt in your building
7. Now your STP should block the old link and trk1 is the forwarding link, just add the old link to trk1 and you got your 7 gb link