Yes, I understand that, participation can happen anywhere. Even though there is no one in Medieval there are other RPGs that you can participate in (see the RPG List page). The reason for this requirement is based on experience where a player joins a game and then takes up to five days just to make an entry into the Camp. This makes the game dull and boring. Thanks for understanding, plus I see from just the posts you have made your activity is back up already.
No, you cannot crossover just like that. Your character has to finish the current scenario, return home and then on a new day happen to meet the players in Party One if possible. If everything aligns and there is room (there is plenty right now) then your character will be moved over.
Nothing to get angry about, however this is already explained:
Spell Casting & in the FAQ: HERE.
Please read the Sticky Threads and / or do a Search and you will find that a lot of the info is available for you.