Taking on window work with no experience
If you want to take on window accounts in addition to your present services, but have no experience, you should hire someone with experience and have them do the work while you supervise them at first.
Windows washers are a dime a dozen on craigslist, and not at all hard to find elsewhere. Plus, window cleaning is a trade that only takes 40-80+ hrs of training before you go out on your own.
By supervising the person you hire, you ensure the quality is up to par, but you'll also have a chance to learn how to wash windows yourself, which really only takes about 40-80 hours of training, with the rest coming from practice on your own. Then you can decide if you want to have your employee do the work, do it yourself, or both.
If you have some money to invest, you could also buy a WFP and use that to supplement your time while your supervising your new employee, who is in effect training you in traditional methods.