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Consistently with the principles of sovereignty of any given government on its own citizens, no government is forced by international law to extradite its citizens to a foreign government. Thus, the French government (as most European governments, along with Canada and Mexico among other countries) cannot extradite a citizen to a foreign country.

But, the possibility of stripping one's person from her French citizenship DOES exists for non-French born citizens, as it does for other governments. The French Code notably gives leeway to the government to strip naturalized French people from French citizenship in case of crimes against France's interests, which can be loosely extended to any terrorist act.

Another option is for the government of the country where the crime took place to ask the French government to put the suspected person on trial on its behalf.

For example, film director Roman Polanski was convicted of rape in the United States in 1977 but fled to France before sentencing. As a French citizen, he cannot be extradited to the United States, but the French government did propose to prosecute Polanski could be prosecuted in France if the US authorities so requested, an offer US authorities declined.

So, I think it would be easy for the French government to extradite a man suspected of terrorist acts and condemned in absentia in a foreign country, but probably not for hate-preaching imams.

For one thing, those are usually not French citizens (hence their deportation instead of imprisonment). For another, I don't see foreign governments bothering with prosecuting them for hate speech only.