You sure can! You have the option to manually enter your fitness stats on both the app and on the website.
On the app, select the ‘Fitness’ button on the profile tab, select the type of activity you want to input from the drop down menu, then choose the day for which you are entering your steps, then tap and hold for 3 seconds. If you choose to manually enter your steps, Spire will stop automatically syncing your steps for the rest of that day. You will have to continue to enter them manually.
On the website, choose ‘Fitness Stats’ from the dropdown menu in the top right corner of your profile page, and input your fitness numbers there.