Use a network Time Machine drive locally
You can try mounting that volume and then use the tmutil command to tell Time Machine where do you want backups to go to. Just make sure that nobody else is using the sparsebundle from the network

Stepwise:


Mount the .sparsebundle disk image
Open Terminal
In the Terminal command line, type:

sudo tmutil setdestination /Volumes/{mounted-disk-image}


Be sure to replace {mounted-disk-image} above to the drive's mount point name.
Open the Time Machine menu and force-start a backup.


Watch the contents of the Backups.backupdb folder in the disk image as Time Machine runs and ensure that it is creating your new backup set.

Note that you'll need Lion for this. Enter the command man tmutil in the Terminal for more information.