Yes it is possible.
For example, in your custom HTML web resource or form, use the following syntax:
<button id="btnOk" class="ms-crm-RefreshDialog-Button" onclick="btnOkClick();" accessKey="O"><u>O</u>k</button>
In the above example, the italic bold code highlighted above is the way to achieve it. Here the user can access that button using Alt + Accesskey(i.e., O).
Hope this helps.