"A pretend conversation" is grammatically correct. People who are inclined to call any word that can be placed before a noun to modify its meaning an "adjective" classify pretend as an adjective when it is used like this; I'm not sure about this classification, but you can think of it that way if you find it helpful. The division between parts of speech can often seem blurry in English; pretend can also be used as a noun.
The online version of the Cambridge Advanced Learner's Dictionary seems to have a definition of pretend as an adjective:
imaginary or not real:
- "Do you want a cup of tea?" she asks, offering me a pretend cup.
- They knew the argument was only pretend, but they still got upset."
Pretend doesn't behave exactly like a typical adjective though.
It can also be used as a noun, as in "they were playing pretend." (I found mention of this in the Oxford English Dictionary.)
I would say it is definitely no more of an adjective than the word fun. Like fun, pretend can be used:
- as an uncountable noun: "You mustn't be afraid of pretend" (1965 G. McInnes Road to Gundagai iii. 54, cited in the Oxford English Dictionary)
- as a predicate after a form of to be: "It was all pretend"
- as an attributive word modifying a following noun: "a pretend conversation" (your example)
Pretend is not like a normal adjective mainly in that it is not "gradable":
Actually, this probably makes it less of an adjective than fun ( "funner" and "funnest" are stigmatized, but have some use, and "very fun" does exist for some people, although other people find it ungrammatical).
Another similar word is make-believe. This also started out as a verb, but is now also used as an uncountable noun, as a predicate after be, and as an attributive "adjective":
- the children made believe they were doctors
- She squandered millions on a life of make-believe.
- The violence in those films was too unreal, it was make-believe.
- 'But, why?' he asked in make-believe astonishment.
(Example sentences from the Collins English Dictionary)