To convert a stakeholder to a persona, open the respective stakeholder and click on the button "Convert to persona".
By the way, the difference between a persona and a stakeholder is that the persona may contain more information than the stakeholder. However you can use both personas and stakeholders in any part of Smaply, does not matter if stakeholder maps, backstage processes, etc.
Using a persona as a stakeholder
If your stakeholders are as thoroughly outlined as any persona, then you could treat them as such within Smaply and use the persona templates to outline your stakeholders as well. We don't have a way to currently delineate them as stakeholders, but you could do that by using the "serious icons" for your stakeholders and "classic icons" for your users.