This seems to test OK. In my case I have assumed that your delimited is actually a comma space, as opposed to just a comma.
so
apples, peaches, pears, plums
would translate to
apples, peaches, pears and plums
You can either make a new field for this based on your current field or if you post the current formula for your list, I can help you with the new formula all in the same formula box, if you need help with that.
If( // first check that we have at least one comma, else nevermind the and Trim(Part([My Comma Delimited List field],2,","))="", [My Comma Delimited List field], // everything up to the last comma NotRight([My Comma Delimited List field],",") // "and" everything after the last comma & " and" & Right([My Comma Delimited List field],","))
