I do not see the commodity that I grow in CropManage. Can I use another one that is similar?

Yes, this is a reasonable way to use CropManage if the commodity that you grow is not available.  For example if you want to use CropManage for irrigation scheduling in pasture or hay you could adapt the alfalfa model.  You may need to readjust the canopy curve or adjust some of the planting settings.   Similarly, if you grow a tree crop that is not available in CropManage you could adapt one of the available tree crops.  If you have questions on how to make some of these adjustments send a message though the feedback link or to mdcahn@ucanr.edu