I think Visio has not such a function.
But if you can work hard with Word Art, it may be possible.
Make Object with word Art in Word, copy and paste it to visio as meta-file
or as Word Picture, ungroup it.
Each fragment of ungrouped shape is a shape that has a shape of character.
This shape can be modified by Pencil tool, and fill color can be changed.
It will also be confirmed that the shape has really a visio geometric shape,
if you open the shapesheet, and if you check the Geometry section.
When you make Word Art, you should select simple black type.
The Ward Art can be made directly on Visio Drawing by the next steps.
1. Menu Insert/Object
2. Select Microsoft Word Picture in the dialog.
3. Select menu Insert/Picture/Word Art.
4. Make Object with WordArt.
5. Close Microsoft Word Picture.