I am not at all an expert in Latin, but I am a Spanish speaker so our languages are a little related, at least. I was wondering if I could just add suffixes to words to make new words. Sometimes, in Spanish, they don't make a lot of sense, so I was wondering if this word made some sense, or if it could at least exist because I didn't find it anywhere on the internet.
Yes, but it's either archaic or incorrect.
In Latin, all nouns follow a declension pattern. You know how verbs in Spanish act differently depending if they end in -ar versus -er versus -ir? In Latin, nouns work the same way.
The word dominus "lord" follows the "second declension pattern", which means that the nominative plural ("lords") and the genitive singular ("of the lord") are both dominī in Classical Latin.
However, in Old Latin, the genitive singular was dominī, and the nominative plural was dominei. (If you go back even further, it was dominoi.) So in Old Latin, dominei means "lords", or "the lords".
Later, ei started being pronounced just the same as ī. Eventually it started being spelled as ī too. But for a while in the middle, people mostly used ei and ī interchangeably. During this time, the genitive singular might also be spelled dominei just by "mistake", like how people sometimes substitute an s for a c in English. While Cicero wouldn't consider this correct, I wouldn't be at all surprised to see it in graffiti.