Now for the tricky part ....

A blender node is used to merge the two textures. The original texture is plugged into Input1, and your patch texture is plugged into Input2. The mask is plugged into the Blending channel, with its value set to 1.

Use the image node's Scale and Offset values to adjust the size and position of the patch texture, so it overlaps the the original texture properly. These values will be different for each clothing item; you'll have to experiment. Set Image_Mapped to "Clamped" to prevent the texture from tiling or repeating.

Your scale/offset settings must be identical for both texture and mask.