Can a solo book be sold to a publisher more easily if it can be spun into a series?
In my opinion, yes. It's much easier to sell a second and a third book by the same author than it is to sell the first book by a new author. Series books ride the coattails of their predecessors and require less work to get them into the stores.
However, don't send a fully outline of an entire series with the submission of your first book. Query the first book, then add one sentence at the end of the letter along the lines of, "I have ideas for serializing these characters."