When you say dynamically, do you mean while everything is actually in motion, or can the machine stop until it changes.
As far as I know, there is no method of changing the configuration of Mach 3 from G Code - except through the known commands e.g. absolute and incremental positioning, or constant velocity mode, or other things.
Unless someone else can tell us, it would seem to me that what you are wanting to do is change the configuration of the spindle from a "spindle" to an axis. I am sure this could be done by writing a Vis Basic Macro, although where I peek and poke the relative information, I do not know.
I did, however, just think of a simpler way (perhaps). Have you tried controlling the spindle as though it were an axis yet.
Assuming that you can - and assuming the spindle stays in the correct position when you have done it, then why not resort to a relay system - a two pole change-over should do - controlled by an output from Mach3 - there are several available.
The machine stops - having been in "spindle" mode. The relay switches from the control wires for the spindle, to the control wires for the C axis - and you then control the c axis for your machining.
At the end, the relay disengages and you are back to a spindle again.
If you can drive your spindle from both types of output then that would seem the simplest way to me -