My protein is under transcriptional control of the T7 promoter system. Can I use it for protein expression in the myTXTL kits?

Yes! That only requires the addition of the plasmid coding for T7 RNA polymerase under transcriptional control of a σ70-specific promoter, e.g. P70a-T7rnap and possibly your inducer like IPTG if it is an inducible promoter. The optimum concentration of P70a-T7rnap is usually between 0.1 nM and 1 nM. Higher concentration normally does not increase protein yield. The more important parameter for efficient protein expression is the concentration of the plasmid that encodes for your protein of interest downstream of the T7 promoter, which will be most likely in the range of 5-20 nM.