I would advise against refilling gas cylinders that are intended to be disposable.

When a container is filled, it is most important that it is not completly full, in order to allow room for expansion.
If the container is overfilled, then even a modest increase in temperature may cause it to burst.

There is also the risk of filling a butane container with propane or with a butane/propane mix which has a higher pressure.

The risks may be somwhat reduced by
Only filling on a hot day, in order that subsequent temperature increases are unlikely.
Only partialy filling
Keeping re-filled containers outdoors
Not keeping them long term.

Refilling should be done, if at all, outdoors due to the risk of leakage.

The best advice though, IMO is dont.

AFAIK in some states of the USA it is a federal offence to transport refilled disposable gas canisters.