can an alloy be used to make another alloy?
An alloy is purely a mixture of metals (and sometimes non-metals) which are non-chemically bonded. 

So yes, you could mix steel and silver to create an alloy. There would be nothing stopping you from doing this, but I cannot say whether or not it would produce any desirable results. Mixing alloys with other materials is merely creating another alloy. Stainless steel is actually just regular steel combined with Chromium.

An alloy is usually created to increase certain properties of its constituents. For example, iron is mixed with carbon in order to create steel, which is far stronger and can be used to manufacture items such as girders. Or, in other cases, Chromium may  be added, if the steel is to be used in, say, cutlery.