On a Mimaki JV33 with white/silver you have to have profiles for the ink configuration loaded, then in Production Manager under the job defaults for the printer you tell it you have the ink set with white/silver.
In FlexiSign itself you would then use the spot color library for your printer. In other words if you had a shape as the bottom layer, select it, then in the fill/stroke editor make sure it is solid fill then click advanced, then library tab, vendor and you can select your machine brand and color library specific for it. You can also add the color library to always display at the bottom of the main screen as well.
However on looking just now - under Roland, I see a metallic library but nothing that is white specific. I'm not familiar with Roland stuff though. On Mimaki there is a specific color library for metallic and another one specific for white and under each you have a lot of mixed shades and colors as well as "raw white" and "raw silver" for when you want to just use a pure spot color.