Can I offer the computer a draw in Phoenix?

You cannot offer a draw. To do so, simply stop the game.
Only in adaptive games (against Shredder) you have to consider your own rating. If you abort a game against Shredder, Shredder himself will rate it as a draw because of the positional rating (< +/- 4). So you can simply abort and start a new rated game. The previous one will not be evaluated as a "surrender" and thus a loss. See also the previous point how the adaptive scoring mechanism of Shredder works.