The additional charges come less than 24 hours after Hasan's civilian lawyer was notified that the army planned to evaluate Hasan's competency to stand trial, as well as his mental state at the time of the shooting.
Retired Colonel John Galligan, Hasan's lawyer, said everything "seems to be on the fast track in this case".
Galligan said he had filed an objection to the evaluation, pointing out that Hasan was still in intensive care recovering from gunshot wounds that left him paralysed.
"I'm incensed at the way the military is handling this, serving additional charges on my client when he's in the hospital and defence attorneys are not present," Galligan told the Associated Press news agency.
"And nobody will tell me what the plans are for the evaluation."
Hasan is accused of killing 12 soldiers and a civilian, and wounding dozens others in the November 5 rampage at the army base in central Texas before he was due to be deployed to Afghanistan.
Investigation and review
The case has drawn criticism after if became known that Hasan had been in contact with a figure sympathetic to al-Qaeda.
US officials have said that intelligence agencies had previously investigated Hasan last December, reviewing what they believed to be email communications between him and a Muslim cleric in Yemen who was known for his anti-American teachings.
Federal officials ended the investigation after concluding that Hasan's communications were related to research he needed to work as a psychiatrist at the medical centre situated inside the US base.
Robert Gates, the US defence secretary, has named two former military officials to lead a review of the Fort Hood case with the aim of preventing a similar attack.
Barack Obama, the US president, has ordered a probe into how US intelligence agencies handled information they may have gathered about Hasan.