I use your overlay with layman and get annoying warning with emerge:

WARNING: One or more repositories have missing repo_name entries:

    /var/lib/layman/scalatools/profiles/repo_name

NOTE: Each repo_name entry should be a plain text file containing a
unique name for the repository on the first line.

It's pretty easy to fix this problem; just add a new directory named "profiles" in repo root, and then put a text file within called "repo_name" which just contains the word "whiter4bbit". I'd do it on my system, but then it'd be overwritten when I sync.

does it possible to add this overlay using layman?

git clone git://github.com/whiter4bbit/overlays.git

Cloning into 'overlays'...
remote: Counting objects: 215, done.
remote: Compressing objects: 100% (110/110), done.
remote: Total 215 (delta 93), reused 188 (delta 66)
Receiving objects: 100% (215/215), 32.29 KiB, done.
Resolving deltas: 100% (93/93), done.

emerge sbt-bin

Calculating dependencies... done!

Verifying ebuild manifests

Emerging (1 of 1) dev-java/sbt-bin-0.13.0_pre1 from gentoo-scala-tools
Downloading 'ftp://mirror.switch.ch/mirror/gentoo/distfiles/sbt-launch-0.13.0-20120530-052139.jar'
--2013-05-04 21:49:12-- ftp://mirror.switch.ch/mirror/gentoo/distfiles/sbt-launch-0.13.0-20120530-052139.jar
=> ‘/usr/portage/distfiles/sbt-launch-0.13.0-20120530-052139.jar’
Resolving mirror.switch.ch... 130.59.10.36
Connecting to mirror.switch.ch|130.59.10.36|:21... connected.
Logging in as anonymous ... Logged in!
==> SYST ... done. ==> PWD ... done.
==> TYPE I ... done. ==> CWD (1) /mirror/gentoo/distfiles ... done.
==> SIZE sbt-launch-0.13.0-20120530-052139.jar ... done.
==> PASV ... done. ==> RETR sbt-launch-0.13.0-20120530-052139.jar ...
No such file ‘sbt-launch-0.13.0-20120530-052139.jar’.

Downloading 'http://typesafe.artifactoryonline.com/typesafe/ivy-releases/org.scala-sbt/sbt-launch/0.13.0-20120530-052139/sbt-launch.jar'
--2013-05-04 21:49:13-- http://typesafe.artifactoryonline.com/typesafe/ivy-releases/org.scala-sbt/sbt-launch/0.13.0-20120530-052139/sbt-launch.jar
Resolving typesafe.artifactoryonline.com... 23.20.223.55
Connecting to typesafe.artifactoryonline.com|23.20.223.55|:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2013-05-04 21:49:13 ERROR 404: Not Found.

!!! Couldn't download 'sbt-launch-0.13.0-20120530-052139.jar'. Aborting.

• Fetch failed for 'dev-java/sbt-bin-0.13.0_pre1', Log file:
• '/var/tmp/portage/dev-java/sbt-bin-0.13.0_pre1/temp/build.log'

Failed to emerge dev-java/sbt-bin-0.13.0_pre1, Log file:

'/var/tmp/portage/dev-java/sbt-bin-0.13.0_pre1/temp/build.log'

• Messages for package dev-java/sbt-bin-0.13.0_pre1:
• Fetch failed for 'dev-java/sbt-bin-0.13.0_pre1', Log file:
• '/var/tmp/portage/dev-java/sbt-bin-0.13.0_pre1/temp/build.log'

The instructions include:

echo "PORTDIR_OVERLAY=$PORTDIR_OVERLAY /usr/local/portage/overlays" >> /etc/make.conf

(1) The file has been moved to /etc/portage/make.conf. Even if this file exists on older systems it is no longer used (unless it's a symlink to ./portage/make.conf).

(2) If there is already an existing PORTDIR_OVERLAY this will break the working system by replacing it with a new one.

A better approach would be:
. /etc/portage/make.conf;
if [ -n "$PORTDIR_OVERLDAY" ]
then
# append whatever you like to the file, or better yet, insert it near the top
# where any existing definitions are already defined.
else
# install into $PORTDIR_OVERLAY
fi

However you like it, the path to make.conf needs to be updated for the instructions to work and some check for an existing PORTDIR_OVERLAY is necessary to avoid breaking existing installations.