Currently, I think that you cannot. What you can do, is to change the default setting for all fonts ( http://plantuml.com/font.html ) but I guess that this is not what you want
To be clear, you need something like:
skinparam CreoleMonospacedFont Consolas
Is this what you are expecting ?
Note that a possible workaround would be to have:
class Foo {
This is <font:Consolas>monospaced</font>
}
Using the preprocessor ( http://plantuml.com/preprocessing.html ) , you can also have:
!define MS(x) <font:Consolas>x</font>
class Foo {
This is also MS(monospaced)
}