Presidential spokesman Farhatullah Babar said the government had already decided in principle that Khar, 34, would be elevated as a federal minister.
"So far, no date for the oath taking has been set but it may take two to three days," he told the media.
The swearing-in ceremony is expected to be held on Wednesday or Thursday as President Zardari is travelling to Afghanistan for a day today.
Prime Minister Yousuf Raza Gilani, before his departure to Britain on a private visit on Monday, advised the President to make Khar the Foreign Minister. Gilani met Khar at the Prime Minister's House before leaving for London and discussed her new role.
Zardari is travelling to Kabul to offer condolences to Afghan President Hamid Karzai on his brother's assassination. He will also discuss bilateral issues and the regional and international situation.
Khar's first foreign engagement after her elevation is likely to be a meeting of the ASEAN Regional Forum to be held in Bali, Indonesia, during July 22-23.
However, some reports suggested that Khar might be made a full-fledged minister after her return from Bali and before her departure for New Delhi for Foreign Minister-level talks to be held at the end of July.