You can. Other people just need your steem username and then can transfer you some steem whenever they like. You can post that you are looking for donations and for what cause and people who support your cause could donate if they wanted to. Even if they just upvote your post, it can be a source of a bit of steem that you can use for your cause.

Of course, you should make it very clear who you are and should verify your identity as an organization and so on so people trust your plea for donations.