If you put the font into the \ui\fonts subfolder under MoI's install location then MoI will be able to use it without needing it to be installed in the OS.

But that's all that should happen by having it in there, it won't change the default usage.

You can set the default font for dimensions under Options > Dimensions > Font and if you want to use it for the UI as well that's in moi.ini under:
[UI]
<...>
UIFontNameV4=Open Sans

- Michael