Oct 25 (Reuters) - Carl Icahn has asked Southwest Gas Holdings Inc (SWX.N) to first offer stock to existing shareholders, if the natural gas transporter goes ahead with a plan to raise up to $1 billion to finance its deal to buy Questar Pipelines. Billionaire activist investor Icahn is trying to gain control of the board of Southwest Gas and replace its chief executive officer, as the company moves forward with the planned Questar acquisition. Icahn has said the deal will hurt Southwest shareholders. The activist investor, who has a stake of just under 5% in Southwest Gas, earlier this month announced his intention to launch a proxy contest to replace the company"s entire board and commence a tender offer at $75 per share in cash. read more Icahn said he plans to file formal documents with the U.S. Securities and Exchange Commission on Wednesday for the tender offer, according to a letter on Monday. (https://bit.ly/3pEG2hJ) Southwest Gas agreed to buy Questar Pipelines earlier in October from Dominion Energy Inc (D.N) for $1.54 billion in cash and assume $430 million in debt.Southwest Gas, which adopted a shareholder rights plan or a "poison pill" a few days later to thwart a push by Icahn to abandon the deal, did not immediately respond to a Reuters request for comment.