You can add custom lines to most graphs in JMP by right clicking the axis and selecting Axis Settings, then manually adding a line in the Reference Lines section.
If you're looking to do this programmatically (you mention that the lines you want to add are variable), this can be done as well. Screenshot and script below.
dt = open ("$SAMPLE_DATA\Big Class.jmp");
var = 145;
Graph Builder(
Transform Column( "Row", Formula( Row() ) ),
Size( 794, 355 ),
Show Control Panel( 0 ),
Variables( X( :Row ), Y( :weight ) ),
Elements( Points( X, Y, Legend( 4 ), Jitter Limit( 0.9541 ) ) ),
SendToReport(
Dispatch(
{},
"weight",
ScaleBox,
{Add Ref Line( var, "Solid", "Red", "Custom Limit", 1 )}
)
)
);