I'm not sure if I fully understand your question so perhaps you could clarify further if I've misunderstood.
Are you trying to specify the connection string for a MS access database at runtime? If your using Server.MapPath to build a physical path to a ms access database from a relative path you can simply use the result of this call as part of the connection string e.g.
DbUtils.ActualConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Server.MapPath("/MyAccessDb.mdb") + ";User Id=admin;Password=;Jet OLEDB:System Database=;Jet OLEDB:Database password=";