Yes, you can combine cash and bonus bets. If you're trying to make a wager greater than the bonus bet, bonus bets and deposited/winnings can be applied to the wager for a greater amount.
For example: If you make a $25 wager and have a $10 bonus bet, you can use the $10 bonus bet and then apply any funds from your wallet to the remaining $15 of the wager you are making for a total wager amount of $25 ($10 bonus bet + $15 cash).