Not sure what you are referring to as a "touch dimmer" switch. A photo would be helpful. You also are not very clear about the existing switch. Is it two switches, side by side (in a two-gang box)? Or is is two switches, one above the other, that take the same space as one standard switch (commonly called a "stacked switch")?
I will take a guess with the limited info anyway.
I am guessing that you have the "stacked switch". If this is true, then you will likely have to cut out the single-gang box and replace it with a two-gang one. Then you will be able to use a standard switch for the fan and the dimmer for the light. The problem is that there is usually a reason for the stacked switch being needed in the first place. Usually, the reason is that the framing did not allow enough room for a two-gang box. The only place that I see this very often is in older mobile homes. If this is the case, you are probably in over your head.
You may be out of luck on this one. I am not aware of anyone making a stack switch with one half as a dimmer. Anyone else know of such an animal?
Whatever you do, please do not connect the fan to the dimmer. You will ruin the fan and, possibly, the dimmer too.