Can I add adjustments to draft accounts without posting them in Xero?

Yes. An adjustment is a balanced draft journal — a debit account, a credit account, an amount and a narration — or an imported multi-line journal. It is overlaid on the figures before the statements are built and is never written to Xero. Adjustments are saved with the draft. The corporation tax estimate from Tax Planning can be offered as a quick-fill adjustment; it travels into the pack labelled as an estimate for planning, never a tax return, and a company whose year end is not the draft's date is listed as not estimated, never as nil tax.