Hi Donnia, 

You would have to say something like -

Tous les mensonges sont intentionnels/délibérés = all lies are done on purpose

or 

quand on ment on le fait exprès = when you lie you are doing it on purpose

But it doesn't flow off the tongue, you would probably say -

Mentir est un acte délibéré = to lie is a deliberate act

But it is fine to say -

Tu le fais exprès de mentir? = Are you lying on purpose?

 

Hope this helps!