Gen Prem said he did not have the answer just now because he did not know who would invite him.
When asked if he thought Prime Minister Yingluck Shinawatra's reform idea would lead to national unity, Gen Prem said he knew nothing about the matter and had no role in it.
A source said Ms Yingluck, in her capacity as the defence minister, will tomorrow lead the permanent secretaries for defence and the three armed forces leaders to pay the elder statesman a visit at his Si Sao Thewes residence.
The source said the premier wanted to meet Gen Prem to wish him a happy birthday ahead of his 94th birthday on Monday.
This would be the first time Ms Yingluck will have met and delivered a birthday message to Gen Prem.
It is a tradition that the defence minister leads the defence permanent secretary and the armed forces leaders to wish Gen Prem a happy birthday.
The source said Ms Yingluck would also talk about her political reform idea when she meets with Gen Prem.
The prime minister, however, would not ask Gen Prem to take part in the reform assembly.
She would seek his views on the reform issue, the source said.
Meanwhile, Democrat spokesman Chavanond Intarakomalyasut said party leader Abhisit Vejajiva has written back to Ms Yingluck declining to take part in the reform assembly.
The prime minister extended the invitation to Mr Abhisit, who has insisted his party will not participate in the assembly's first meeting tomorrow.
He said the Democrats could not join the forum for several reasons. There was a lack of clarity about the assembly's ultimate goal, while the government was also being insincere by refusing to withdraw the controversial amnesty bill from parliament, he added.
The government's political reform process looked more like a "political drama", he said.
PM's Office Minister Varathep Rattanakorn said the Democrats should reconsider as the assembly will be open to a free flow of ideas and suggestions.