The precise structure of Buffett's participation is not yet clear and discussions are ongoing, the paper said, quoting people familiar with the transaction.
Some of the people said it would be in the form of preferred shares, and one said Buffett's company Berkshire Hathaway would provide about 25 per cent of the deal's financing.
The companies announced the plan on Sunday and the Journal said a final deal valued at around 10 billion dollars or more could be announced in a day or two.
The two companies' current market value is about $18 billion.
They said Sunday the new group would be traded and based in Canada, where the merged company's largest market is located.
A Canada home base will also permit the company to benefit from Ottawa's more favorable tax code.
Buffett could conceivably take flack for that because he has been outspoken in his position that the ultra-rich in the United States should pay more in taxes.