This may be a tricky situation. If you have already routed the board then you can try and add the net in the Schematic inside Multisim and then select TOOLS>Forward Annote to Ultiboard 10. You will get a message about overwritting the previous file. This is natural. Also when Ultiboard retrieves it there will also be a message that importing a new net list will erase the old one. This is also normal.
After Ultiboard imports the new net list will require you to route the new connection either manually or with the autorouter. If you are adding new components in this manner then that will require you to place the component since Ultiboard does not automatically place components inside the board outline on a forward annote.
This gets tricky when you have already routed the board, especially if you are removing something from the original schematic. This situation would call for you to remove open ended coppper traces using the EDIT>Copper Delete>Open trace Ends. Then sometimes when that doesn't work you have to just bite the bullet and remove all copper and re-route.
I hope this works for you mand doesn't cause a mess with your layout. If worst comes to worst you can always remove all copper and re-route.
