You can use a service like Windows Live Writer to add content to your Squarespace blog, in addition to posting directly from your Squarespace site.
Step 1: Setup your blog within Windows Live Writer
Within Windows Live Writer, navigate to Blogs > Add blog account.
Step 2: Setup your blog information
After selecting Add blog account, the Add blog wizard will walk you through setting up your Squarespace account.
Add your Squarespace site URL, enter the owner login and password for your Squarespace site, and then click Next.
Note: If you have multiple journal pages added to your site and you would like to post entries to each one, you will need to setup a Windows Live Writer blog account for each journal page. Additionally, you will want to enter the actual URL for each journal page in the Web address of your blog field (instead of your main site URL).
To post to the site using an editor account, you will need to enter the login for the editor as follows:
The password will be the normal editor password to log into your site.
Choose Other blog service from the list of services, and click Next.
In the drop down menu, Select Metaweblog API as the blog type. Add this as the remote posting URL:
Once this information is filled in, click Next.
Windows Live Writer will then connect to your Squarespace journal page and make sure all settings are correct. You will then be asked to setup a Nickname for your blog. This will default to the page name for your journal. Click next when complete.
Windows Live Writer will now be fully configured, and you can post your first entry.
Step 3: Posting an entry to your blog
Once Windows Live Writer is fully setup, you will then see an add new entry screen. You can add the title and add the content of your entry within this page.
Once complete, you can click Publish to publish the entry to your Squarespace journal.
Note: Squarespace does not currently support tagging posts using Live Writer -- you must log in and modify the entry directly to add tags.