It could be an interesting house rule as there has been no version of the rules that states that the numbers of pieces of your color is intended to restrict your maximum military size.
If exceeding your pieces is allowed, simply choose a spare color, if there is one, or designate some other kind of playing piece from another game, or otherwise improvise.
It is worth checking all players have the same number of pieces available to make this fair.
In the classic “World Domination RISK" game of military strategy, you are battling to conquer the world. To win, you must launch daring attacks, defend yourself on all fronts, and sweep across vast continents with boldness and cunning. But remember, the dangers, as well as the rewards, are high.
See this RISK instruction leaflet to learn gameplay variations and variations for RISK experts.