* will it add change addresses to my wallet?
As o_e_l_e_o said above, if you imported via seed phrase, then yes.
* can I use one of my existing addresses as a change address?
To use a specific address as a your change wallet you can use the "send to many" feature. In the "send to" field you can list multiple addresses, followed by a comma, then the amount you want to send. Use a exclamation point for the last address, which will be your change address. The exclamation point will select the remaining amount, minus fees to send to that address.
Example:
1xFirstPaymentAddressxxxxxxxx,0.0035
1xSecondPaymentAddressxxxxx,0.004
1xSpecificChangeAddressxxxxxx,!
Electrum will show you the outputs before you confirm them, and you can double check the addresses there. If the change address is in your wallet it'll be highlighted in green. The change addresses created by Electrum will be highlighted in yellow.