I wouldn't recommend it...
That said... I looked all over for instructions how to accomplish this and if Alex had instructions on his site, it looks like he's removed it. The level of features in more recent versions of Motion and FCPX far outreaches the current version of iMovie.
Facts:
There are no external locations for which templates are saved for iMovie as there are for FCPX (for which there are at least 2, not counting importing into Libraries.)
The only location I could find is:
iMovie (> Show Package Contents) > Contents > Plugins > MediaProviders > MotionEffect.fxp > Contents > Resources > Templates /
Inside there are folders for Effects, Generators, Titles and Transitions.
All of these contain themed projects*.
Most of them are "dated" (versioned**) at displayversion 5.0.5. One that I found is set to 5.1.9.1 (there's no such version released to the public - which goes from 5.1.2 to 5.2). This version (5.2) corresponds to Final Cut 10.2.0, so there is likely NO 3D Text support. You will also not be allowed to use any feature of Motion that was added from this time.
Version 5.0.5 (Motion and the XML) corresponds to FCPX 10.0.6.
Any template you would add to iMovie would need to be compatible with FCPX 10.0.6 or earlier! (Motion 5.0.5)
If you should have a template from this time (or successfully create and backdate a template) then you could try to install the *themed* template into the iMovie application (make a backup of your original in case anything goes wrong).
*A themed project is created from within Motion upon Saving. Or, you can edit the XML file which will require adding:
[Motion 5.0.
<template>
<flags>1</flags>
</template>
or
<template>
<theme>ThemeName</theme>
</template>
[spacing and indentation count! That's a 4 space indent.]
to the XML project file just before the <build></build> tag line (with a blank space between the </template> tag and the build line).
Themes are an extra level of "category" in the installation process:
.../Templates/Effects/Category/Theme/Project
** Backdating a project can be done by also simple edits to the Project XML file (all the Motion project types are simply XML / OZML text files — I recommend using BBedit or TextWrangler if you have it). Instructions for backdating a Motion project can be found here:
https://fcpxtemplates.com/fcpx-motion-compatibility/
Best of luck! I'd like to hear if you are successful and how the templates actually work in iMovie (parameters, keyframing and such).