IF you have or have access to a 3D-printer than you can print a respooler like this
I designed a bulk film respooler to help convert 400ft rolls of film to the more commonly available 100ft spools. This was designed to be as small as possible so it can fit in your dark bag. UPDATE:Experimental Version 2.1 - 400ft-100ft BASE ONLYVersion 2.0 without sidewallsVersion 1.0 with...
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There are several models on the net
The cheapest way ist to put the film spool into a darkbag and wind it onto a second core by hand with feeling and patience.
Lay the spools flat on the table, put on cotton gloves and wind the film on to the other core.
When both spools seem to have the same diameter then cut it.
To be on the safe side make three spools out of one so that the new spools will safely fit into the bulk film loader.