The simplest method is to co-culture with the new host at high and low MOIs and recover the progeny phage. Of course, this COULD result in MONVALENT phages specific for the new host.
Here is a short n sweet paper that describes this:
Competition and the origins of novelty: experimental evolution.......
Lisa M. Bono, Catharine L. Gensel, David W. Pfennig and Christina L. Burch
Biol. Lett. published online 17 October 2012
The other more 'genetic' method is to modify the tail fibers used for attachment to the host. By choosing appropriate sequences - for example, those that attach to a ubiquitous surface protein - would change or broaden host specificity. However, this alone may not be sufficient since host restriction/modification systems can limit host replication.