After having searched a lot, I still didn't find an option to align multirow cells vertically correctly in Lyx' native tables. There seem to be some solutions but I haven't found any for use with Lyx native tables without individually adjusting the vertical displacement.
I created an example file to show what I mean and the corresponding rendering. The code in Lyx is as followed:
\begin{table}
\begin{centering}
\begin{tabular*}{1\textwidth}{@{\extracolsep{\fill}}>{\raggedright}m{0.2\textwidth}>{\raggedright}m{0.4\textwidth}>{\raggedright}m{0.3\textwidth}}
\toprule
\addlinespace
\textbf{Typ} & \textbf{Technologie} & \textbf{Materialien}\tabularnewline\addlinespace
\midrule
\addlinespace
Extrusion & Fused Deposition Molding (FDM) & Thermoplaste (z.B. PLA, ABS), HDPE, eutektische Metalle, Gummi, Ton,
Knetmasse\tabularnewline\addlinespace
\midrule
\addlinespace
Draht & Electron Beam Freeform Fabrication (EBF) & fast jede Metalllegierung\tabularnewline\addlinespace
\midrule
\addlinespace
\multirow{5}{0.2\textwidth}{Granulate } & Direct metal laser sintering (DMLS) & fast jede Metalllegierung\tabularnewline\addlinespace
\cmidrule{2-3}
\addlinespace
& Electron-beam melting (EBM) & Titanlegierungen\tabularnewline\addlinespace
\cmidrule{2-3}
\addlinespace
& Selective laser melting (SLM) & Titanlegierungen, Kobalt-Chrom-Legierungen, stainless-steel, Aluminium\tabularnewline\addlinespace
\cmidrule{2-3}
\addlinespace
& Selective heat sintering (SHS) & thermoplastisches Pulver\tabularnewline\addlinespace
\cmidrule{2-3}
\addlinespace
& Selective laser sintering (SLS) & Thermoplasten, Metallpulver, keramische Pulver\tabularnewline\addlinespace
\midrule
\addlinespace
\multirow{2}{0.2\textwidth}{Pulverbett mit Inkjetkopf} & Plaster based 3D printing (PP) & Gips mit Wasser als Binder\tabularnewline\addlinespace
\cmidrule{2-3}
\addlinespace
& kein einheitlicher Name findbar & Sand/Kunststoff mit Binder\tabularnewline\addlinespace
\midrule
\addlinespace
Laminiert & Laminated object manufacturing (LOM) & Papier, Metallfolie, Plastikfilm\tabularnewline\addlinespace
\midrule
\addlinespace
\multirow{2}{0.2\textwidth}[-2mm]{Photoaktivierte Polymerisation} & Stereolithographie (SLA) & \multirow{2}{0.3\textwidth}{Photopoylmere}\tabularnewline\addlinespace
\cmidrule{2-2}
\addlinespace
& Digital Light Processing (DLP) & \tabularnewline\addlinespace
\bottomrule
\end{tabular*}
\par\end{centering}
\protect\caption{Übersicht über verschiedene additive Fertigungsverfahren in Anlehnung
an \cite{wikipedia20143Dprinting}. \label{tab:=0000DCbersicht =0000FCber verschiedene additive Fertigungsverfahren}}
\end{table}
Focus on the word "granulate"

You can download the working example here: Working Lyx example
In this question here, they showed how to use the second optional argument but isn't there an automatic way?

Granulatein the first row of the area that it should span via a\multirow, place it in the third row, then it will be centred automatically without using a\multirow. – Werner Jul 05 '14 at 15:31