It is important that you describe exactly what size and shape this "very small" aluminum part has. Since you state that you wish to make the tool in steel, I'm assuming you will be using some process such as die casting or possibly even squeeze casting. It should be no problem to have your steel mold designed and manufactured if you have a CAD model of the part, and also have sufficient financial resources since making even a small steel casting tool can be quite expensive. If you are using a process like die casting or squeeze casting, it is usually best to hire an expert tooling engineer to design your mold since it is no simple thing to make these tools function properly.
If all you require is a few dozen examples of your small aluminum component, you should consider using some rapid prototype process like metal laser sintering.
Good luck to you.
