I do that in my e-mail client (outlook express)
by using a copy of my ISP e-mail account and changing the e-mail address to what I need at my site.
I export the ISP account to save it,
modify it with the new address
and discription like
send as me@my site from ISP
then I import the backup of the ISP account
and make it the default to send from.
When I need to send as ME,
I just select it in the drop down menu at the top of the new e-mail where it says From:
So I do not need the actual accounts on my site.
Not all ISP's will allow this , but try it.