Here I go, leaving unpopular clarifications again...
Short answer is NO, but it is complicated...
The question being answered here is "Is it possible to "charge up" a sorcerer/warlock by taking multiple short rests?".
~The assumption is the Coffeelock combo where you turn your Warlock Spell Slots into Sorcery Points, then your Sorcery Points into Sorcery Spell Slots, short rest for 1 hour to regain Warlock Spell Slots, then rinse and repeat. Stacking up to 8 hours worth of spell slots to use during your adventuring day.
There are a couple good reasons why this does not work.
Can work, but not indefinitely
A.) Short Rest and Long Rest are mutually exclusive.
This is backed by a JC tweet confirming that the intent was that you cannot take short rests and long rests at the same time. A long rest does not necessarily need to contain sleep, as there are species that simply do not sleep, like Elves, and Warforged. Also any Warlock with Aspect of the Moon also does not sleep. But they still need to long rest. As an excerpt of Xanathars Guide to Everything mentions.
Whenever you end a 24-hour period without finishing a long rest, you must succeed on a DC 10 Constitution saving throw or suffer one level of exhaustion.
It becomes harder to fight off exhaustion if you stay awake for multiple days. After the first 24 hours, the DC increases by 5 for each consecutive 24-hour period without a long rest. The DC resets to 10 when you finish a long rest.
With all that said, you would at this point be able to perform the base actions of a Coffeelock for a period or time before you become too exhausted to carry on, or need to take a night off to restart the long rest cycle. But there is more to elaborate on with regards to this consideration.
General Vs Specific kills it.
B.) Warlock Spell Slots are not eligible to be turned into Sorcery Points.
Another JC Tweet, as mentioned above by "Tommi" in a comment specifies that as intended the wording of the Multiclass and Spellcasting section of the PHB is intentionally excluding Pact Magic from other Spellcasting Classes. The wording is:
Pact Magic. If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.
With that in consideration, your Pact Magic Spell Slots are separate from your Spellcasting Spell Slots and thus would not be viable with the Flexible casting feature. With this in mind you would most certainly be unable to use your Warlock Spell Slots to convert them to Sorcery Points. Which eliminates the popular Coffeelock combo from being viable, answering the question above as definitively as I can.
The reason your Pact Magic Slots and Spellcasting Slots are separate with this reading is that it specifies that you can use your Pact Magic slots for casting spells. It does not say you can use your Pact Magic slots interchangeably as spell slots in general, just to cast spells.
Font of Magic from the Sorcerer Class does not specify where the slots need to come from. It generalizes slots as its source. But considering the rule of specific vs general; because the multiclassing rules specify you can use your Pact Magic Slots for casting spells from other spellcasting classes, and not that you can use your Pact Magic Slots interchangeably as Spellcasting Slots, they would be ineligible as slots accessible to the Sorcerers Font of Magic feature.