rather than doing a postscript at the end of your email just add: By the way, Ms._________, I would greatly enjoy maintaining contact through email. I have always appreciated your dedication to your students and I have truly appreciated all which I have learned throughout my course of time in your class. Hope to remain in touch.
Yes it is. If the above is too much just write her with:
P.S I would love to stay in touch through means of email. I thank you for everything you have done this year and hope you have a wonderful summer vacation.
I'd just put "PS. Feel free to use this email address if you want to get in touch" and leave it at that.