You'll need an inline (mounted on cable, not in a cabinet) 4-pole plug (it looks like the standard Yaesu plug, so any place like DigiKey should stock them) and an inline 3.5mm (1/8") stereo jack (Radio Shack carries them). It looks as if one of the poles (rings or the tip) of the plug is for the mic, the other two are for speakers (or earphones) and the shell is ground. Your speaker plug is shell=ground and tip and ring are each a speaker.
Any high school kid who's taken an electronics course should be able to figure out which pin is which on the earphone plug (plug the unconnected new plug into the jack with one earphone wire grounded and probe the other 3 poles for sound. The two that have sound are the speaker/earphone poles.) 5 minutes and about $15 and you're good. (The 4 pole plugs, while standard, are still a bit costly. The RS jack should be about $4 (which is a ripoff, considering that they used to sell them, made in the USA, for about 69 cents for 2).
There's one problem if the speakers aren't amplified. The earphone output of a cellphone is deliberately kept very low, so you can hear but you can't damage your hearing. That's about enough that, when fed to a speaker, may just be loud enough for you to hear if you put your ear up against the speaker. That's why there are amplified speakers and why there are Bluetooth speakers (which, IMO, are better - they're smaller, produce pretty good quality sound, and you don't have to worry about wires. And they won't cost much more than the parts to make an adapter.)