Virtually everyone carries a cup - use it to transfer water from the source to the Platy.
Personally, I carry a 1 liter wide mouth soda bottle in addition to my 2 liter platy hoser system inside my pack. I fill the soda bottle, add the aquamira, and hike on while it treats, drinking from the platy system as needed.
At meal stops, or on those very rare occasions when I exhaust the platy before reaching another water source, I refill the platy from the now treated soda bottle, then refill and treat the soda bottle ASAP. Just walk them over, so to speak.
If I know there's a very long dry stretch ahead or a dry camp between me and the next water source, I'll fill all or part of both platy bladders (I carry a rolled-up 3 liter bladder for this) and the soda bottle for a total of 6 liters. I can always dump excess water in the morning bfore starting out if I don't use as much as I thought in camp and overnight. Works every time.
CAVEAT: everyone's water needs are different over the identical ground. My wife drinks like a sponge when I can go 10 miles on a liter. KNOW YOUR BODY. Listen to it and learn or suffer the consequences. Remember, there is no such thing as a 100% guarenteed water source. Eventually you will reach a planned "year-round dependable: water source only to find it dry or otherwise unuseable.
Survival secret - carry a plastic soda straw. It allows you to drink from a very shallow depression in a rock or from a thin layer of water flowing over a rock.