I need to know if as a sportsman can say:
I am going to dope?
I need to know how (especially) the Americans use the verb dope.
Your sentence is perfectly understandable. To my ear, the most natural use of the verb "to dope" is in the gerund form as in the following examples,
Mike, are you doping?
The goalie for the US women's hockey team was found guilty of doping.
You certainly can use I dope, you dope, he dopes etc. But it isn't as common in every day language.
In your context I would probably say,
Should I use steroids? [or whatever more specific drug to which you are referring]
I am going to use steroids.
An athlete, not a sportsman. Though technically synonymous, they're not, not really. Henry Higgins was a sportsman, according to Alfred Doolittle, anyway.
"I am going to dope?" may be wrong depending on the context. It is a question.
"Am I going to dope?" is correct but extremely awkward.
"Would I dope?" or "Should I dope?" (depending on the context) would be more in keeping with standard English.