"I would like to keep my Tiscali email address but wonder if this is possible."
That depends entirely on Tiscalis Ts& Cs. You need to read them/ask Tiscali. You may need to pay for the privilege.
You may find that you cannot send via Tiscali s servers unless you have an account with them - again you need to ask. Many ISPs prohibit this.
When you have two email addresses/accounts, you find that OE has a "From" bar at the top, with a drop-down arrow at the right end. This allows you to choose which outgoing account to use.