I understand that the minimum part size for uploading to an S3 bucket is 5MB
Is there any way to have this changed on a per-bucket basis?

The reason I'm asking is there is a list of raw objects in S3 which we want to combine in the single object in S3. 

Using PUT part/copy we are able to "glue" objects in the single one providing that all objects except last one are >= 5MB. However sometimes our raw objects are not big enough and in this case when we try to complete multipart uploading we're getting famous error "Your proposed upload is smaller than the minimum allowed size" from AWS S3.

Any other idea how we could combine S3 objects without downloading them first?