Can I add uber earnings from a previous year if they weren't previously reported?

Income received in a prior tax year cannot be entered on the current year tax return.  You will have to amend the prior year tax return that was filed to add the new income.

See this TurboTax support FAQ for amending a prior year tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-already-filed/L4VjJ9BA2_US_en_US?uid=ld1n99kr

No, as DoninGA stated, you will have to file an amended return to correct the income that was excluded.  Income is reported to the IRS for the tax period it is earned in.  The IRS cross references these reporting with what is on your tax return.  If you do not correct the tax return, the IRS will often issue a notice making the changes for you and charge penalties and interest on the income that was missing.  

For more information see the link below:

• Will the IRS Catch missing income?

@tbibbs86