Since banks are not in the real estate business but in the lending and financial solutions business, a Realtor will not be able to negotiate directly with the bank on your behalf regarding a foreclosed home that has not yet been listed for sale.
Here is what a good agent will do for you. Begin the market analysis to provide you with an opinion of market value so that you are ready to present an offer immediately when the home is listed. The two of you can discuss all of the terms you will want in the purchase contract so that the offer can be prepared quickly with only a few updates needed based on any possible requirements that will be ultimately published with the listing.
Often when the property is assigned to an agent, they will put a business card or sign in the window of the home (as an emergency contact) prior to listing. If you see such a sign, alert your agent as to the name on the sign and they can call the agent and request the selling agent call them when the property is going to be listed.
In some MLS systems a buyers agent can also set up an auto search for a specific address. Any listing or changes to the listing will trigger an email to the agent and/or their client. So you will find out within a matter of hours when a listing goes live on MLS.