For MP, do you mean a xxx.waypoints like this?:
QGC WPL 110
0 1 0 16 0 0 0 0 33.74113111 -118.374600 178 1
1 0 10 22 0 0 0 0 33.74116111 -118.374600 1.00 1
2 0 10 201 0 0 0 0 33.74111111 -118.375 1.00 1
3 0 0 93 0.001 0 0 0 0 0 0 1
4 0 10 16 0 0 0 0 33.74111111 -118.37472222 4.00 1
5 0 10 16 0 0 0 0 33.74117111 -118.37472222 1.00 1
6 0 10 16 0 0 0 0 33.74124111 -118.37472222 4.00 1
7 0 10 16 0 0 0 0 33.74117111 -118.37472222 1.00 1
8 0 10 16 0 0 0 0 33.74111111 -118.37472222 4.00 1
9 0 10 16 0 0 0 0 33.74105111 -118.37472222 1.00 1
10 0 10 16 0 0 0 0 33.74099111 -118.37472222 4.00 1
11 0 10 16 0 0 0 0 33.74105111 -118.37472222 1.00 1
12 0 10 177 3 3 0 0 0 0 0 1
13 0 0 93 0.001 0 0 0 0 0 0 1
14 0 3 16 0 0 0 0 33.74111111 -118.37472222 20.00 1
15 0 10 16 0 0 0 0 33.74117111 -118.37472222 1.00 1
16 0 3 16 0 0 0 0 33.74124111 -118.37472222 20.00 1
17 0 10 16 0 0 0 0 33.74117111 -118.37472222 1.00 1
18 0 3 16 0 0 0 0 33.74111111 -118.37472222 20.00 1
19 0 10 16 0 0 0 0 33.74105111 -118.37472222 1.00 1
20 0 3 16 0 0 0 0 33.74099111 -118.37472222 20.00 1
21 0 10 16 0 0 0 0 33.74105111 -118.37472222 1.00 1
22 0 3 177 13 3 0 0 0 0 0 1
23 0 10 21 0 0 0 0 33.74119111 -118.374600 0.00 1
You can modify it with a column selection editor (Textpad, Geany…) or with a spreadsheet. If the first column gets messy, copy and paste it from one generated in a spreadsheet. Other columns to change manually may be the waypoint type, or the coordinates, copying and pasting them with column selection.
I think that in MP you get a similar result if you create one and use something as save as, but for manual modifications you can go this way.
MP is ported to Linux and MacOS, but it is not very comfortable to use.