The Support Phase (Rulebook Chapter 9)
"During the support phase of the active players turn, he may assign up to one single dice from his reserves as worker by moving it to his support zone. Assigning dice this way is a special action that has nothing to do with dice already in play. Once assigned – the die serves a different purpose for ..."
The new rulebook blogpost explains how to assign dice as workers by moving them from reserves to the support zone.