It is certainly OK to use a mono mike with a stereo recorder. But there are several ways to connect the two, and once you decide how you want to proceed you will need the proper cable.
The simplest approach is to connect the mono mike to one channel of the stereo deck. If the deck has separate input jacks for left and right inputs, then all you have to do is plug your mono mike into one channel or the other. If the deck has just a single stereo input jack, then you will need a splitter adapter, or a specially wired cord.
The other option is to feed the single from the mono mike to BOTH channels of the stereo deck. Here it's easier if the recorder has a single stereo input.
You can also buy yourself some protection against overload if you feed the mike signal
into one channel directly, and split it to the other channel through an attenuating
resistor. This way you can use the recording from the left channel, but if you've
overloaded the left channel you can use the right channel which was recorded X dB
lower.