yes, you can absolutely do that. You can draw the model in Onyx and never connect it to any data (I sometimes use the parameter names to put any „label“ on paths I want for teaching, even nonsense stuff like ‚look here: this is now smaller‘ :). You can even use your MPlus code to have Onyx create a graph for you; it’ll probably not look very nice, but if you drag the variables around to a design you prefer, that usually results in something nice. Onyx won’t read MPlus code directly, but you can go via lavaan without problems: lavaan accepts the same code as MPlus (but is free and on the more general platform of R), and then you can call Onyx with a lavaan model. I have to say that personally, I rather prefer do redraw the graph myself, because then I can arrange them as I like from the beginning, but both options should work!
In either case, you can use the starting values to put any value you want on the paths manually if you don’t want to load your data into Onyx.