No, you need to pay them a fixed rate. It is allowed to pay commission on top of a fixed rate, but you have to hire people on a fixed rate or hourly basis. A lot of people will probably avoid such projects though, as there's no guarantee they'll get paid as you can't really escrow commission, and if it was done hourly, then commissions wouldn't qualify for the hourly guarantee.
I'm sure you're a genuine guy who'll pay up, but one of the down sides of freelancing on platforms like Upwork is the whole anonymity of the people you're working with... escrow and hourly guarantees help to minimise the risk of not getting paid, and it's not advisable to work on projects that don;t involve one or the other.
Just my thoughts on the situation.
Commissions, in addition to hourly pay or fixed-price payments, can be paid as bonuses using the Upwork client-side interface. There is no limitation on the amount of money you can pay contractors in the form of bonuses.