You can use either balanced or unbalanced cables to connect your amp's direct output to your interface's input.
Balanced ins and outs are for controlling noise on longer cable runs, and they are compatible with unbalanced cables and connections. Using an unbalanced cable simply does away with the noise-cancelling signal but retains the original signal and shielding.
One thing you'll probably have to consider when combining direct and mic'd signals is you might need to flip the phase of one of the signals, adjust your mic position, or fiddle with the phase alignment in your software. This will help compensate for the mic'd and direct signals arriving to "tape" at slightly different times.