Is it a good idea to add an extra word to the seedphrase? Like a really easy word that I won't forget, so someone that came across my seed (and knows what those words are) wouldn't be able to just open his phone and steal my funds right away.
It is a good idea to add an extra word, phrase, or string for additional security, but I wouldn't make it a "really easy word". If someone performs a simply dictionary attack, then they could brute force it in a matter of seconds. At the very least I would choose a few words, and add some additional numbers or symbols to increase the entropy. As with all passwords, ideally you would want to use a completely random string, something along the lines of
["N8&#L9@~.c+\yp, but I also realize most people don't do that. Backing it up on paper separately from your seed phrase is a must.
