Yes, you can and there’s even a bulk upload feature to help.
If you’re using LC Email (what is LC Email?), then you can only upload contacts that have previously opted-in to receive info from you (more details here).
If you’ve connected your own Mailgun account or other 3rd party SMTP Provider to Growably, you can upload contacts that you have rented or purchased from elsewhere (again, more details here).
However, in both cases, before you do…
Make sure you have completely cleaned, verified & scrubbed the list first.
(yep, I had to bold and underline that last line as it is vitally important 
)
If you don’t, you will ruin your email domain sending reputation with all the mail providers when they see you sending to email addresses that don’t exist anymore / bounce etc.
It can be VERY hard to rebuild a bad email domain reputation.
Thankfully, there’s some VERY easy to use tools that you can run your email list through before uploading to Growably, such as Validity by BriteVerify, & Never Bounce.
Tools like these are VERY cheap and VERY easy to use.
All you need to do is upload your contact list into them, click a few buttons and pay a few bucks.
They’ll then automatically verify every email on the list and a few minutes later you’ll have an updated / verified list of emails with all the old/junk/bouncing emails removed so you can safely upload it to Growably.
IMPORTANT
Do not send any emails through Growably before reading this article and configuring your preferred email / SMTP Service.