Hi,
I am not for sure whether I got your intention.
Generally, LXQt and SDDM are two different processes on a system. LXQt is the desktop environment (which is based upon the toolkit Qt) while SDDM is the Display Manager. On one hand, you can use SDDM also with KDE or GNOME desktop environments, and on the other hand, you can use LXQt also with LightDM.
So, it is not important which desktop environment you are using: you can use SDDM with all (at least all widespread) desktop environments. So the answer to your question is generally yes.
Depending on the distro, it is usually very easy to change the display manager. Usually, this an be done using the distro’s package manager (e.g. dnf, yum, apt). So for example, “dnf install sddm”. That’s it.
After installing, you have to disable the old manager on your system, and then to enable the new one. This is to be done using the command line tool “systemctl” which controls systemd. It is very well documented on the internet.
Maybe this will be enough to fulfill your needs. If you want to copy the config as used on Fedora LXQt, you will find it in /etc/sddm/.
But be aware that after logging into your account, your desktop environment (e.g. LXQt) is started as user process and is then controlling most “things” that are displayed. These “things” are often independent applications. This includes the user’s lock-screen which can be part of a screensaver application like XScreenSaver (which is configured in /etc/xscreensaver/). So the LXQt configuration in your home directory is maybe a more interesting starting point: ~/.config/lxqt/
My Fedora is using SDDM with LXQt and XScreenSaver. Lock-screens are provided by XScreenSaver on my system but to be honest, I do not remember whether this is Fedora default or custom 
Specific themes are maybe part of further packages, Fedora’s dnf is listing the following xscreensaver packages:
============================================================================ Name Exactly Matched: xscreensaver =============================================================================
xscreensaver.x86_64 : X screen saver and locker
=========================================================================== Name & Summary Matched: xscreensaver ============================================================================
xstar-xscreensaver.x86_64 : XScreenSaver support for xstar
qstars-xscreensaver.x86_64 : XScreenSaver support for qstars
tempest-xscreensaver.x86_64 : XScreenSaver support for tempest
xscreensaver-base.x86_64 : A minimal installation of xscreensaver
xscreensaver-base.x86_64 : A minimal installation of xscreensaver
xmountains-xscreensaver.noarch : XScreenSaver integration support
================================================================================ Name Matched: xscreensaver =================================================================================
xscreensaver-extras.x86_64 : An enhanced set of screensavers
xscreensaver-gl-base.x86_64 : A base package for screensavers that require OpenGL
rss-glx-xscreensaver.x86_64 : Really Slick Screensavers
xscreensaver-gl-extras.x86_64 : An enhanced set of screensavers that require OpenGL
xscreensaver-extras-gss.x86_64 : Desktop files of extras for other screensaver
xscreensaver-extras-base.x86_64 : A base package for screensavers
xscreensaver-gl-extras-gss.x86_64 : Desktop files of gl-extras for other screensaver
Regards,
0xc3