This might be too loaded, but I'll give it a try:
عندما أتكلم الفصحى أحتوي عليها العامية المصرية
أتكلم الفصحى و(أنا) مزج معها العامية المصرية
To add to Elroy's comment, I thought I'd let you know what's incorrect in أحتوي and the أنا :
a7tawi means: I contain, which wouldn't make sense in this sentence. To use your sentence, you can say something like:
عندما أتكلم الفصحى أخلط بها كلمات عامية
(just as an example, there are other possibilities)
The problem with وأنا is simply that it's redundant. If you didn't use the pronoun in the begining of the sentence, why did you put it in the middle? Remember that the conjugation of the Arabic verbs usually denotes by itself the subject of the verb. So you simply say:
أتكلم الفصحى وأمزجها بالعامية
or
أتكلم الفصحى ممزوجةً بالعامية
And mamzuja(tan) would be the حال of الفصحى . And you can replace ممزوجة with مختلطة .
In the spirit of
this thread maybe we could make up our own portmanteaux:
These were fun, Josh

But I hope you won't really use them, or else you'd need ennak tewazza3 ma3ahom catalogue besh-shar7
