here's the Wiki article on the Rose Cross.
I read that already, the problem is that it's too confusing for me

I need a simplified version...
Well, regarding the time dilation ability, currently, it's impossible because of the canon convention that typical esper abilities do not influence the human body in any way, shape, or form. Exception is Auto Regeneration, but it falls under the Special Level 0 class so it can be overlooked. So direct manipulation of the human body is impossible in canon sense.
But with Auto Regen it does open up a world of possibilities. Think about what it would take to recover from massive internal bleeding and organ failure. Then there's Konori whose eyes can be altered to see X-ray. Also, Accelerator can use his power to speed up his metabolic rate to heal his wounds. Though it was not mention how long it would take... I suppose the time it would take flying from Japan to Russia.
So it is possible, but you have to draw a line. We can't have people changing their DNA and morphing a blade-arm, or suddenly growing spikes out of their body, or changing the carbon in their skin to become a diamond armor
>.>.