That is of course possible. I suppose you are using toolbar/menu mode and not the ribbon mode as you wrote toolbar and not ribbon.
Right click on any currently visible toolbar and click in context menu on item Customize...
You have two options now. You modify one of the predefined toolbars or you add a new toolbar with your custom commands. I suggest the latter as it is better on upgrades. So select second tab Toolbars, click on button New and type a name you want for the new toolbar not yet existing for any other toolbar. The new toolbar is immediately displayed without any symbol in the middle of the screen. Move the new toolbar to the position you want among the other toolbars using drag and drop.
Select tab Commands and select category Edit. There is the command Spell check in the alphabetically sorted commands list of this category. Drag and drop this command over the new toolbar or an existing toolbar if you want to modify a predefined toolbar. The vertical bar being displayed shows where the command is inserted in the toolbar on leaving the mouse button.
You might want to add also the command Spell as you type to predefined or new toolbar. This command is in category Miscellaneous (and of course also in category All commands). This command toggles the spell as you type feature and gives you also a visual indication if that feature is currently enabled or disabled. I use the feature spell as you type very often which is enabled/disabled by me always using a hotkey which I assigned to command Spell as you type at Advanced - Settings or Configuration - Key mapping. I have the command nevertheless in my custom toolbar to see on the symbol the activation status of this feature.
Best regards from an UC/UE/UES for Windows user from Austria