OK. If you're going the manual way (which I guess several of us are going to advise against, but let's answer first) you can basically type in the text on the right part of the editor, provided you have loaded the right table.
Alternatively, if the game uses a well-known table, you could open a new text file in your favorite editor (notepad++ comes to mind), set the editor to use that well-known encoding (say: SJIS, UTF8 or ASCII)n input the text in the file, save, then use binary insert of Windhex from that text file.
Jut don't forget that there's nothing preventing you from overflowing the old text's location, overwriting the next text in the process, when you do things this way. Also, you're probably limited to the lengths of the old texts (either individually or as a sum).
This is why people generally recommend to dump from the text pointers (not the text), and go the same route for insertion. But either that is a simple set of pointers and you can yse Atlas + Cartographer, or you need custom tools.