You can.
Necropotence fills your hand on your end step, which despite the name occurs before the "until end of turn" that YawgWill looks for. There is a window to play spells after taking them with Necropotence yet before discarding; and YawgWill continues its effect until after discarding.
Just as a general note, once you start discarding however, unless there is a triggered ability or SBE that would go on the stack, no one gets priority.
313. End of Turn Step
313.1. As the end of turn step begins, all abilities that trigger “at end of turn” go on the stack. (See rule 410, “Handling Triggered Abilities.”) Then the active player gets priority and players may play spells and abilities.
313.2. If “at end of turn”-triggered abilities are created or if cards with “at end of turn”-triggered abilities come into play after preexisting ones have already gone on the stack at the beginning of the end of turn step, those abilities won’t go on the stack until the next turn’s end phase. In other words, the step doesn’t “back up” so new “at end of turn”-triggered abilities can go on the stack. This only applies to triggered abilities that say “at end of turn.” It doesn’t apply to continuous effects whose durations say “until end of turn” or “this turn.” (See rule 314, “Cleanup Step.”)
314. Cleanup Step
314.1. If the active player’s hand contains more cards than his or her maximum hand size (normally seven), he or she discards enough cards to reduce the hand size to that number (this game action doesn’t use the stack).
314.2 Simultaneously, all damage is removed from permanents and all “until end of turn” and “this turn” effects end (this game action doesn’t use the stack).
314.3 If the conditions for any state-based effects exist or if any triggered abilities are waiting to be put onto the stack, the active player gets priority and players may play spells and abilities. Once the stack is empty and both players pass, another cleanup step begins. Otherwise, no player receives priority and the step ends.
