No, you can not use an opponent's ball as the first ball in any shot, including shots on the 8 ball. This act, alone, is a "wrong ball first" standard foul.
For a shot to be legal, the cue ball must first contact a ball from your own group (either solids or stripes) or the 8 ball, if that's the ball you're currently trying to pocket.
The above points apply regardless of whether you are shooting at a regular object ball or at the 8 ball.
So you have certainly committed a foul by hitting the wrong ball first, but the penalty depends on whether you succeed in pocketing the 8 ball or not.
In your scenario;
- if you fail to pocket the 8 ball - it is just a "wrong ball first" standard foul, resulting in the opponent taking the cue ball in hand from anywhere on the table.
- if you pocket the 8 ball - it is a loss of game (because if you make any foul while pocketing the 8 ball, you lose the game).
Here are the specific World Pool-Billiard Association 8 ball rules which relate to this scenario (emphasis ours).
WPA 8 ball rule 3.9 tells us that contacting the wrong ball first is a "standard foul", and the penalty is that play transfers to the opponent with the cue ball in hand anywhere on the table.
3.9 Standard Fouls (8 Ball)
If the shooter commits a foul, play passes to his opponent. The cue ball is in hand, and the incoming player may place it anywhere on the playing surface. (See 1.5 Cue Ball in Hand.)
The following are standard fouls at eight ball:
- (6.2) Wrong Ball First - The first ball contacted by the cue ball on each shot must belong to the shooter’s group, except when the table is open. (See 3.4 Open Table / Choosing Groups).
The "wrong ball first" foul is further explained in section 6.2:
6.2 Wrong Ball First
In those games which require the first object ball struck to be a particular ball or one of a group of balls, it is a foul for the cue ball to first contact any other ball.
WPA 8 ball rules 3.10 Serious Fouls tells us that the player loses the rack if committing any offense listed under 3.8 Losing the Rack, which includes committing a foul while pocketing the 8 ball.
3.10 Serious Fouls (8 Ball)
The fouls listed under 3.8 Losing the Rack are penalized by the loss of the current rack. [...]
...and the list of rack-losing offences:
3.8 Losing the Rack (8 Ball)
The shooter loses if he
- (a) pockets the eight ball and fouls.;
- (b) pockets the eight ball before his group is cleared;
- (c) pockets the eight ball in an uncalled pocket; or
- (d) drives the eight ball off the table.
These do not apply to the break shot. (See 3.3 Break Shot.)