Why the offset to ADC ? Are you using + input to ADC from a bridge
element, a little better description of why you need the offset would
help forum assist you.
In general, offsetting from a reference would make most sense to
preserve absolute accuracy, if thats what you are seeking.
You can use a VDAC for giving offset of 2.048V or a Vref component followed by PGA of gain 2.
Keep in mind the onboard reference is accurate to +/- .1% and the
PGA gain error is > +/- .15%, its offset error is 10 mV, and its PSRR
is 48 db, so factor those into your error analysis.
Using a battery is something that i would suggest, if you are looking for a higher resolution(higher ENOB).
If you are OK with a resolution less than or equal to 16bits, then you can seek the help of references and DACs as the preconditioning circuit always reduces the ENOB.
A battery is not a panacea for fixing a reference problem, as it has
a noise contribution, its noise rises with depletion, and usually one
needs a R network to get the reference voltage of interest which also
contributes to ENOB reduction.
Conduct a complete signal chain analysis and that will reveal the issues.
Best is to try out all possibilities and select which is suiting your requirements.