A list of different ways to deal with veggies that need more cooking time during a stir fry, in order to end up with evenly-cooked vegetables:
The stagger method, which most home cooks use — put veggies that need to cook longer in the wok first. Wait a few minutes, and put the next set of veggies in.
Cut those veggies into thinner strips than the others, since thinner ingredients cook faster.
Blanch them. Compared to parboiling, this likely results in less water-logging, which is good because moisture inhibits Maillard reactions.
A non-traditional method — microwave them. Often the goal of stir frying is to encourage the Maillard reaction, and microwaves target water molecules (and a few other substances). Microwaves also heat up the surface of an ingredient more than its interior, which again enhances Maillarding.