I have a number of fruit trees that have been in the ground for several years. I did not have access to mycorrhizal innoculants at the time of planting. Can I inoculate them somehow now? How?
Why do you think you need to add mycorrhizae? Are the trees having problems?
If you planted into healthy soil to begin with, and especially if you amended the soil in your planting hole, you probably already have beneficial fungi working with your trees.
You could mulch around your trees with a half inch of compost and then a couple of inches of arborist wood chips -- out to the drip line -- and you'd end up with mycorrhizae on the soil surface. But compost and mulch are a good idea anyway beyond whatever benefits you might get from the fungi. (Just don't "volcano mulch", and don't mulch right up to the trunk.)