Ok, so it's a physical disk allocation and not a "merge attachments with another directory".
The simple answer is that there's no simple way to do this. JIRA has an attachments directory at a single point and you need to present that to the system at a particular place for it to work.
Now, although it's a bad thing to use a network or shared disk for most of JIRA's home directory (very bad, trust me), it is ok to do it for the attachments. What we often see on medium/large sites is either symbolic linking or disk mounts for the attachments directory - those both locate the attachments on a separate disk.
You could do the same at a lower level, if one of your projects is taking 90% of the attachment volume, mount a new disk at /attachments/<project key>, but that's not really ideal. The best answer here would be to move the contents of the attachments directory on to a new (network) drive and mount or link that to replace the old attachments directory.
For example, a JIRA I've got has a simple symbolic link to remote mounted storage at
/opt/jira-home/data/attachments -> /mnt/extern3/attachments
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