charging small nimh packs

put a 250 ohm resistor in parallel
You'll get a VERY hot resistor! Work out how many watt you will be dumping through it! You would also need to put a diode between the resistor and the cell-pack, or it could discharge thorugh the R if you weren't careful
Perhaps you meant series?
How many cells in the pack? A 6v input might just be enough volts to charge a four-cell.
It's a long time since I tried charging a nimh, but If I recall, they don't like a constant current charge, and you need some way of measuring end voltage, so the input tapers off as it is reached.
A proper nimh charger is only a couple of quid on fleabay.

Is it a welded pack, or four loose cells?
A normal Futaba (etc) 4 cell flight pack charges ar 50 mah x 14 hrs

Charging the 250mHr pack at the recommended maximum of 10% would be 25mA. Assuming your charger charges at 6volts, if you put your 250ohm resistor in series for charging a dead flat battery you would get an initial charge rate of 24mA dropping to 8mA as the battery passed through 4volts. I would say you'll be fine and you will never damage these new batteries of yours.

welded packs not loose cells - - sorry, missed that!

www.google.com/search?client=firefox-b-d...+from+volts+and+amps

Power = Volts² / Resistance, so, suppose you're dropping 6 volts across the resistor P=6x6/250 = .144watts so a small 1/4watt resistor will do the trick. You must remember that resistors are quite fragile so if you could find a wire wound of that size it would be better, but still beef up the ends of the body with some silicone sealer to allow you some lead flexibility.

Ouch! That's a lot of money!

Most of that will be p+p! Small resistors cost about 2p each, sold in 10's